(1) Field of Art
The present invention relates to a structure of a gas component detecting plug superior in water resistance and productivity.
(2) Proir Art
For taking out as an electrical output component gases such as combustible gas and oxygen remaining in exhaust gases discharged from various combustion devices, including internal combustion engines and blast furnaces, there has widely been known a gas component detecting plug which employs a gas component-sensitive element (hereinafter referred to simply as "gas-sensitive element") formed of a semiconductor comprising a metal oxide such as TiO.sub.2.
Typical examples of this type of gas component detecting plugs include one in which an insulating tube with a gas-sensitive element attached to one end face thereof and with electrode wires connected to the gas-sensitive element and projecting from the opposite head end face through through-holes is sealed to an enclosure, and one in which an insulating board with a gas-sensitive element and electrodes formed on the surface thereof by a thick film printing and interconnected at an end portion by soldering or the like is sealed to an enclosure. In these conventional gas component plugs, in deciding axial or longitudinal and radial or transverse positions of the insulating tube or board (hereinafter referred to simply as "insulator") relative to the enclosure, first the insulator is inserted into the enclosure and then is fixed in a predetermined position by the use of a heat-resisting cement. Further, moisture contained in exhaust gases may get into the interior of the enclosure from the gas-sensitive element side (hereinafter referred to as "leg portion side") and water such as rain water or stagnant water may permeate from the external circuit side, thereby causing a defective insulation especially between electrode wires. To prevent this, there has been adopted a method in which after hardening of the heat-resisting cement, glass is filled into the space surrounding the lead wires from an opening formed in an end portion of the enclosure on the side (hereinafter referred to as "head portion side" ) opposite to the leg portion side, then a tube formed of rubber, Teflon or the like is inserted and the enclosure is caulked in a radial direction from the outer periphery of the tube. But, such conventional gas component detecting plug involves drawbacks. First, it is difficult to insert the insulator accurately in a predetermined position within the enclosure and to pour the heat-resisting cement uniformly into the gap between the insulator and the enclosure. Secondly, the position of the insulator relative to the enclosure must be temporarily fixed by a mechanical means until the heat-resisting cement hardens, which operation is troublesome. Thirdly, since the heat-resisting cement is poured from the heat portion side opening, it inevitably adheres to the end portion of the insulator on the head portion side and also to the electrode wires, so that the space to be filled with glass in the subsequent step is narrowly limited and a perfect hermetic seal can no longer be expected.